Lower critical solution temperature group

Figure 19.1 shows the temperature-dependent volume-phase transitions of NIPA-based hydrogels. It is suggested that formation of a complex between bioactive molecules and functional groups of hydrogels is responsible for immobilization of the former. The lower critical solution temperature (LCST) of PNIPA can be tuned to the required value by introducing hydrophobic or hydrophilic fragments. 

At present, we believe that the jump transitions observed in many of the gels studied here represent first order phase transitions. If this is the case, then the gels studied here are among the first found so far in which a first order phase transition occurs near room temperature in pure aqueous solvent with substantial added salt. Early studies by Tanaka s group with poly(acrylamide) based gels required that hydrophobic solvents such as acetone be added for a discontinuous phase transition to be observed near room temperature [6-10]. The more recently studied gels based on poly(n-isopropylacrylamide) [11, 12] and other lower critical solution temperature polymers show discrete phase transitions in water with no salt [11], but the swelling transitions become continuous when moderate amounts of salt are added [12],... 

Poly(ferrocenylsilanes) with methoxyethoxyethoxy or quaternary ammonium groups on silicon are hydrophilic or soluble in water and display lower critical solution temperature (LSCT) behavior—again, a similarity to polyphosphazenes26 3233 (see Chapter 3). 

Thermoresponsive acrylamide co-polymers were also used to alter the physicochemical and biopharmaceutical properties of avidin. Similar to PEG, the acrylamide co-polymers with a lower critical solution temperature (LCST) of about 37 °C were conjugated to the protein amino groups. The polymers were conjugated either by polymer multipoint attachment using polyfunctional polymers or by single chain attachment using end-chain monoactivated polymer. In both cases, the polymer conjugation was found to produce bioactive derivatives with reversible thermal character (Fig. 11.12). 

As an impressive phase study, it was observed that a blend system composed of CAB having 54 mol % butyryl side groups and polyfethylene oxide) (PEO Mn = 35 000) yielded a phase diagram with a lower critical solution temperature (LCST) boundary, and a cloud point of 168 °C at the critical composition of CAB/PEO = 40/60 (w/w) [112]. 

That crosslinking has indeed occurred is confirmed by the very existence of aggregates at 25 °C, as in its absence the diblock copolymers are completely soluble at this temperature. Tunability of the solubility of the PMEMA block in water arises from the fact that its lower critical solution temperature (LCST) lies between 25 and 60 °C. This reversible hydration of the core could be a very useful feature to trigger release of occluded guest molecules from the core interior. More recently, utilizing a similar methodology, zwitterionic shell-crosslinked systems have also been prepared wherein the core and shell domains contain amine and carboxylic acid groups, respectively, or vice versa. Such systems exhibit an isoelectric point, at a pH wherein the crosslinked micelles ( 40 nm) become electrically neutral and precipitate out in water addition of acid or base causes complete redissolution of these nanospheres [58]. 

There are several different methods to separate PNIPAM-supported catalysts from the reaction mixtures. Both liquid-solid separations and liquid-liquid separations can be used. The most frequently used liquid-solid separation method takes advantage of the varying solubility of polymers in different solvents. For example, PNIP AM can be precipitated from THF into hexanes. PNI-PAM copolymers also exhibit lower critical solution temperature (LCST) behavior. Specifically, PNIPAM and its copolymers can be prepared such that these polymers are soluble in water at low temperature but precipitate when heated up. This property may be used as either a purification method or a separation tech-nique.[l 1] A thermomorphic system is a liquid-liquid biphasic system developed in our group. It uses various solvent mixtures with temperature-dependent miscibility to effect separation of catalysts from substrates and products, as shown in Figure 2. 

The hydrophobic interaction results in the existence of a lower critical solution temperature and in the striking result that raising the temperature reduces the solubility, as can be seen in liquid-liquid phase diagrams (see Figure 5.2a). In general, the solution behaviour of water-soluble polymers represents a balance between the polar and the non-polar components of the molecules, with the result that many water-soluble polymers show closed solubility loops. In such cases, the lower temperature behaviour is due to the hydrophobic effects of the hydrocarbon backbone, while the upper temperature behaviour is due to the swamping effects of the polar (hydrophilic) functional groups. 

LIP Li, P.-F., Wang, W., Xie, R., Yang, M., Ju, X.-J., and Chu, L.-Y., Lower critical solution temperatures of thermo-responsive poly(A -isopropylaciylamide) copolymers with racemate or single enantiomer groups, Polym. Int, 58, 202, 2009. 

Water-soluble hyperbranched PEAs are nowadays developed for different applications such as paper coatings, kinetic hydrate inhibitors (KHIs) for the oil and gas industry, and biology and medicine devices. The last apphcations require a lower critical solution temperature (LCST) around physiological conditions, whereas high LCST values may be useful as KHIs. KeUand [50] has recently demonstrated that PEAs based on a cychc anhydride and diisopropanolamine can be effectively tuned by varying the hydrophobicity of the cyclic anhydride or by the addition of a less hydrophilic secondary amine (i.e., without hydroxyl groups). 

Polymer chains with both hydrophobic and hydrophilic groups interact with water to make organized structures of water molecules around the hydrophilic groups. This lowers the energy of the system, which is typical at lower temperatures. However, at higher temperatures, these water structures breakdown as the water molecules tumble more chaotically, and the polymer chains are forced together to make a solid material. The temperature at which the polymer chains precipitate out of the solution into a solid mass is the lower critical solution temperature (LCST). 

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